Added explicitly-typed lambdas
Remove variable reduction rule

Finding bound variables which haven't been substituted yet should be an error
Change syntax for variables since it clashes with the 'optional' metasyntax
Take out level constraint production since it's not used [yet]
Footnote about variable syntax
Replace explicit annotation syntax with 'as'
Remove distinction between type parameters and indices
New syn/chk arrows, again
Note about Kleene star
Remove reduction from typechecking and add 'equality modulo reduction' rule
Fixes to notation and to elaboration
Lots of fixes to semantics